This invention relates to an apparatus for measuring a reflection characteristic of a sample using an integrating sphere, which may be adopted in a spectral colorimeter.
Generally, measurement of a reflectance of a sample is greatly affected by a configuration of an illuminator and a light receiving device (hereafter, referred to as "geometric configuration"). Accordingly, the Commission Internationale de l'Eclairage (CIE) recommends to use a reflection characteristic measuring apparatus with any one of the following geometric configurations as a reflection characteristic measuring apparatus such as a spectral calorimeter:
45/0: the illuminator is so arranged as to illuminate the sample surface with light incident upon the sample surface at 45.degree., and the light receiving device is so arranged as to receive light reflected from the sample surface at 90.degree.; PA1 0/45: the illuminator is so arranged as to illuminate the sample surface with light incident upon the sample surface at 90.degree., and the light receiving device is so arranged as to receive light reflected from the sample surface at 45.degree.; PA1 d/0: the illuminator is so arranged as to illuminate the sample surface with diffused light, and the light receiving device is so arranged as to receive light reflected from the sample surface at 90.degree.; PA1 0/d: the illuminator is so arranged as to illuminate the sample surface with light incident upon the sample surface at 90.degree., and the light receiving device is so arranged as to receive diffused light.
Among the above configurations, d/8 type (combination of diffused-light-illuminator and +8.degree.-inclined-light-receiving-device), a variation of the d/0 configuration, has been widely used because it can measure both a reflectance of a specular component included reflection light (or SCI spectral reflection) and a reflectance of a specular component excluded reflection light (or SCE spectral reflection). The SCI spectral reflection is unlikely to be influenced by the surface condition of the sample and hence has measurement reliability, and the SCE spectral reflection is close to visual sense.
However, there has been the problem residing in the conventional measuring apparatus that they lack the interchangeability of measurement data among apparatus even if the apparatus of the same geometric configuration are used. One reason for this is that despite the use of apparatus of the same geometric configuration, the apparatus do not coincide, in a strict meaning, with each other in terms of a spectral characteristic and a geometric configuration. It has been difficult to correct the reflectance in connection with wavelength and half bandwidth. It has been more difficult to make the geometric configuration of apparatus agree with each other with a high precision.
The measuring apparatus of d/8 configuration is incorporated with an integrating sphere for diffusely illuminating a sample. Even if the size of the integrating sphere used in the apparatus and the measurement area of the sample are identical, the illumination characteristic to the sample surface slightly differ from apparatus to apparatus. This is another hindrance against pursing the interchangeability of measurement data between apparatus.
Problems involved in pursuing the interchangeability of measurement data are described with reference to FIGS. 2A, 2B, and 3.
1 Although an integrating sphere is adapted for diffusing light, the distribution of illumination light (hereafter, referred to as "light distribution") in integrating spheres differs from one another because their respective physical constructions and their respective reflective characteristics on inner surface are different from one another. Io in FIGS. 2A and 2B represents diffuse illumination light. The diffuse illumination light Io in FIG. 2A has a light distribution represented by a transversely expanded oval where light radiating in the -8.degree. direction with respect to the normal line to a surface of a sample 3 is weak relative to other directions. On the other hand, the diffuse illumination light Io in FIG. 2B has a light distribution represented by a vertically expanded oval where light radiating in the -8.degree. direction with respect to the normal line to the sample surface is strong relative to other directions. It is presumed that the total light amount of the diffuse illumination light Io in FIG. 2A is identical to that in FIG. 2B.
In the case where the sample has a luster surface in FIGS. 2A and 2B, light reflected from the sample 3 consists of diffuse reflection light Rd and specular reflection light Rs. The diffuse reflection light Rd is free from influence of the diffuse illumination light Io. Accordingly, it can be estimated that the light distribution and the light intensity of the diffuse reflection light Rd in FIG. 2A are identical to those in FIG. 2B. However, since the specular reflection light is subject to the light distribution of the diffuse illumination light Io, the specular reflection light Rs in FIG. 2B has stronger light in the +8 direction than that in FIG. 2A due to the light distribution of the diffuse illumination light Io in FIG. 2B.
Accordingly, what is observed on reflected light R.sub.8 in the +8.degree. direction in FIGS. 2A and 2B is that the light intensity of specular reflection light Rs, which does not show wavelength dependency or is free from the influence of wavelength, is strong relative to diffuse reflection light Rd, which exhibits wavelength dependency or is greatly affected by a wavelength peculiar to the sample, in FIG. 2B compared to in FIG. 2A. This causes a spectral reflectance difference between the samples in FIGS. 2A and 2B.
2 Referring to FIG. 3, a beam of light F of a light source 92 is introduced in an integrating sphere 91 and undergoes a multiple reflection on an inner surface 91a of the integrating sphere 91. Part of diffuse light To illuminates a sample 3, and this light is reflected on the sample surface. Part of diffuse light Mo is incident on an optical fiber 93 as reference light or monitor light.
Since the sample 3 and an incident end of the optical fiber 93 are located at different positions, the diffuse lights Io and Mo have a certain proportional relationship (Io.infin.Mo) although not identical to each other. The proportional coefficient established between the diffuse lights Io and Mo varies merely depending on the geometric configuration of the integrating sphere.
Reflected light Ior which is reflected from the sample 3 is diffusely reflected on the inner surface 91a of the integrating sphere 91, and a part of the reflected light Ior illuminates the sample 3 again as diffuse light Is. In illuminating the sample 3 again, a part of the reflected light Ior is incident on the optical fiber 93 as reference diffuse light or monitor diffuse light Ms.
Hereafter, the diffuse lights Io and Mo are respectively referred to as "initial illumination light Io" and "initial monitor light Mo", whereas the diffuse lights Is and Ms are respectively referred to as "second illumination light Is" and "second monitor light Ms".
The second illumination light Is and the second monitor light Ms also have a certain proportional relationship which varies merely depending on the geometric configuration of the integrating sphere. In this time, however, the sample 3 itself becomes a light source for second illumination light. The sample 3 (or light source) is located at a position different from the light source 92. Consequently, the proportion coefficient established between the second illumination light Is and the second monitor light Ms is different from that between the initial sample light Io and the initial monitor light Mo. Further, the ratio of second illumination light to the whole illumination light varies in accordance with the reflectance of the sample 3. Accordingly, the illumination light cannot be accurately monitored or detected. The second illumination light proportionally rises with an increase in the reflectance (r) of a sample. Accordingly, measurement error due to variations in the reflectance is proportional to the square of the reflectance (r.sup.2).
FIG. 4 shows calibration of a measuring apparatus with a white reference sample having a reflectance of 90% to correct such measurement error. In FIG. 4, the horizontal coordinate denotes a true reflectance of a sample and the vertical coordinate denotes an apparent reflectance or measured reflectance. The straight broken line represents an ideal state in which the true reflectance and the apparent reflectance match with one-to-one correspondence. The solid curve represents actual measurement results outputted from the measuring apparatus under the influence of second illumination light. The dashed curve represents calibrated results. As shown in FIG. 4, the calibrated curve is shifted down compared with the apparent reflectance. Accordingly, it will be seen that there is a likelihood that a calibrated measurement is lower than the true reflectance.
The measurement error due to second illumination is also caused by the structure in the vicinity of an aperture for a sample. As shown in FIG. 3, generally, an integrating sphere is provided with a buffer plate 94 near the sample aperture to prevent the sample 3 from being directly illuminated by the light from the light source 92. In this case, it is highly likely that light reflected from the sample 3 is reflected by the buffer plate 94 which in turn re-illuminates the sample 3. The position and the size of the buffer plate 94 is liable to differ from one integrating sphere to another. This difference is another cause for measurement error despite the use of measuring apparatus of the same type. Measurement error due to the difference in the position and size of buffer plate rises in proportion to the square of reflectance of a sample.
U.S. Pat. No. 5,384,641 discloses a measuring apparatus, as shown in FIG. 5, to correct the measurement error due to light distribution difference mentioned in the section 1. In FIG. 5, this measuring apparatus uses an integrating sphere 100 with illuminators 110, 120. The respective illuminators 110, 120 illuminate one after another illumination zones 117, 127 defined in different areas on an inner surface of the integrating sphere 100. The diffuse light distributions of the illuminators 110, 120 differ from each other.
The diffuse illumination light from the illuminators 110, 120 individually illuminates the sample 3, and obtained measurements are properly weighted and linearly combined so as to provide a reflectance approximate to a reflectance under a predetermined light distribution. This is performed to eliminate the measurement error due to light distribution difference pointed out in the section 1. However, this measuring apparatus cannot eliminate the measurement error due to second illumination which is pointed out in the section 2.